No, because the moon takes about 27 days to fully orbit which means that 3 weeks would not be enough to see all of the moon's phases, but if you think that you don't need 27 days due to the fact that some of the days there may not be a phase change then that is completely up to you.
Explanation:
Hope this helps:)...if not then sorry for wasting your time and may God bless you:)
